Vibrating sensors, such as for example, vibrating densitometers and Coriolis flowmeters are generally known, and are used to measure mass flow and other information for materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450, all to J. E. Smith et al. These flowmeters have one or more conduits of a straight or curved configuration. Each conduit configuration in a Coriolis mass flowmeter, for example, has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit(s), and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or a small “zero offset”, which is a time delay measured at zero flow. As material begins to flow through the flowmeter, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the flowmeter lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pickoffs on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pickoffs are processed to determine the time delay between the pickoffs. The time delay between the two or more pickoffs is proportional to the mass flow rate of material flowing through the conduit(s).
Meter electronics connected to the driver generate a drive signal to operate the driver and determine a mass flow rate and other properties of a material from signals received from the pickoffs. The driver may comprise one of many well known arrangements; however, a magnet and an opposing drive coil have received great success in the flowmeter industry. An alternating current is passed to the drive coil for vibrating the conduit(s) at a desired flow tube amplitude and frequency. It is also known in the art to provide the pickoffs as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current which induces a motion, the pickoffs can use the motion provided by the driver to induce a voltage. The magnitude of the time delay measured by the pickoffs is very small; often measured in nanoseconds. Therefore, it is necessary to have the transducer output be very accurate.
Generally, a Coriolis flowmeter can be initially calibrated and a flow calibration factor along with a zero offset can be generated. In use, the flow calibration factor can be multiplied by the time delay measured by the pickoffs minus the zero offset to generate a mass flow rate. In most situations, the flowmeter is initially calibrated, typically by the manufacturer, and assumed to provide accurate measurements without subsequent calibrations required. In addition, a prior art approach involves a user zero-calibrating the flowmeter after installation by stopping flow, closing valves, and therefore providing the meter a zero flow rate reference at process conditions.
As mentioned above, in many vibrating sensors, including Coriolis flowmeters, a zero offset may be present, which prior art approaches initially correct for. Although this initially determined zero offset can adequately correct the measurements in limited circumstances, the zero offset may change over time due to a change in a variety of operating conditions, mainly temperature, resulting in only partial corrections. However, other operating conditions may also affect the zero offset, including pressure, fluid density, sensor mounting conditions, etc. Furthermore, the zero offset may change at a different rate from one meter to another. This may be of particular interest in situations where more than one meter is connected in series such that each of the meters should read the same if the same fluid flow is being measured.
In marine industry applications, seafaring vessels often employ fuel switching schemes, whereby a marine engine operates on different types of fuel (or a blend thereof). Typically heavy fuel oil (HFO) and either marine diesel oil (MDO) or marine fuel oil (MFO) are the fuels used. When the fuel source is switched, the HFO operating temperature of between about 120-150° C., is changed to an operating temperature of about 30-50° C. for MDO/MFO. As there is about a 50° C. temperature difference between the two operating temperatures, temperature-driven zero-drift issues arise.
Therefore, there is a need in the art for a method to determine and compensate for changes in the zero offset of vibrating sensors that experience changes in operating temperature. The present invention overcomes this and other problems and an advance in the art is achieved.